The present invention relates generally to an exhaust gas recirculation control for an internal combustion engine, particularly for a diesel engine. More specifically, the invention relates to an exhaust gas recirculation control for controlling the exhaust gas recirculation rate in precise correspondence to variation of the engine operating condition for emission control.
"DATSUN 810 DIESEL SERVICE MANUAL", published on March, 1981 by Nissan Motor Co., Ltd., as publication No. SMlE-910SUO, shows an exhaust gas recirculation control for a diesel engine on pages EC-2 to EC-8. This discloses an exhaust gas recirculation control for controlling formation of NOx emission and includes an exhaust gas recirculation control valve, a throttle valve with a throttle diaphragm, and solenoid valves for controlling the exhaust gas recirculation valve and the throttle diaphragm. In this exhaust gas recirculation control, the exhaust gas recirculation rate is selected from three allowable stages, namely a HIGH EGR stage in which the exhaust gas recirculation rate is high, a LOW EGR stage in which the exhaust gas recirculation rate is low and a ZERO EGR stage in which the exhaust gas is not recirculated at all. The HIGH EGR is obtained in the combination of closing the throttle valve and open the exhaust gas recirculation valve. The LOW EGR is attained by opening the throttle valve and opening of the exhaust gas recirculation control valve. The ZERO EGR is attained by closing the exhaust gas recirculation control valve.
For precise control of the exhaust gas recirculation rate in accordance with engine operating conditions, three-stage variation of the exhaust gas recirculation rate is not enough. Particularly, it would be desirable to adjust the exhaust gas recirculation rate in strict correspondence to engine operating conditions in the LOW EGR stage. Therefore, according to the present invention, the throttling rate of the exhaust gas recirculation control valve is controlled by control signal which in more detail controls the duty cycle of valve actuator of the exhaust gas recirculation control valve.